LETTER pubs.acs.org/acscatalysis
Selective Amine Oxidation Using Nb2O5 Photocatalyst and O2 Shinya Furukawa, Yasuhiro Ohno, Tetsuya Shishido,* Kentaro Teramura, and Tsunehiro Tanaka* Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
bS Supporting Information ABSTRACT: Niobium oxide (Nb2O5) can function as a photocatalyst for selective oxidation of benzylamine rather than other semiconductor metal oxides such as TiO2, ZnO, and WO3. Various amines including primary, secondary, and cyclic amines are also photocatalytically converted to corresponding imines in excellent yields by using Nb2O5 in an atmospheric pressure of O2. Nb2O5 exhibits a catalytic activity with high selectivity even under visible light (λ > 390 nm) irradiation, although Nb2O5 does not absorb visible light. KEYWORDS: amine oxidation, photocatalyst, niobium oxide, selective oxidation, aerobic oxidation
O
xidation of amine to imine is an important chemical transformation because of the versatile applications of imines as synthetic intermediates of medicines or biologically active nitrogen containing organic compounds.1 Several oxidation procedures using stoichiometric oxidants such as 2-iodoxybenzoic acid2,3 or N-tert-butylphenylsulfinimidoyl chloride4 have been reported. However, a catalytic system using molecular oxygen as a sole oxidant has been desired in view of green chemistry.5,6 In this context, a number of transition-metal catalyzed aerobic oxidation systems have been developed. Ru-based catalysts such as RuCl3,7 [RuCl2(RCH2NH2)2(PPh3)2],8,9 Ru-porphyrin,10 Ru-hydroxyapatite,11 Ru2(OAc)4Cl,12 and Ru/Al2O313 are known to be effective for aerobic oxidation of amines. Au nanoparticles supported on Al2O3,14,15 CeO2,14,15 graphite,16 and hydroxyapatite16 are also found to be good catalysts for amine oxidation. But in these systems, expensive precious metals are employed and relative high temperature (mostly >373 K) is required. Utilizing semiconductor photocatalysts for aerobic oxidation of organic molecules has practical advantages of economical efficiency, environmental-friendliness, reusability, and durability. In addition, to effectively utilize solar energy, it is necessary to develop a material that will function under visible light.17 Very recently, Su and co-workers reported that mesoporous graphite carbon nitride (mpg-C3N4) can work as effective photocatalyst to activate O2 for the selective oxidations of benzylic alcohols and amines with visible light.18,19 Although this material exhibits excellent catalytic performance under visible light irradiation, high oxygen pressure (0.5 MPa) and trifluorotoluene as solvent are necessary to obtain good yields. Zhao et al. reported that photooxidation of amines using TiO2 with UV light gave a high selectivity to imines under a diluted condition.20 We recently reported that photocatalytic oxidation of various alcohols proceeded selectively over niobium oxide (Nb2O5) under a mild condition.21,22 Nb2O5 showed higher selectivities to partial oxidation products; therefore, it can be thought that Nb2O5 is more suitable for selective oxidation than TiO2. Moreover, we found that Nb2O5 can catalyze the selective photooxidation of alcohols even under visible light r 2011 American Chemical Society
Table 1. Oxidation of Benzylamine over Various Metal Oxidesa
entry
catalyst
yield (μmol)
yield (μmol/mmol cat)b
selectivity (%)c
1
TiO2
886
708
89
2
ZnO
790
643
94
3
Nb2O5
649
915
98
4d 5
Nb2O5 MoO3
n. d. 328
472
97
6
CeO2
264
497
97
7
Ta2O5
201
444
96
8
ZrO2
175
216
95
9
WO3
143
332
89
10
V2O5
322
298
59e
11
none
199
81
a
Reaction conditions: catalyst (100 mg), benzylamine (5 mmol), benzene as a solvent (10 mL), irradiation time (5 h), oxygen pressure (1 atm). b Amount of N-benzilidene benzylamine per mole of catalyst. c Selectivity to N-benzylidene benzylamine. Benzaldehyde was yielded as a main byproduct except entry 10. d In the dark. e Benzylamine-N-carbaldehyde was formed as a main byproduct.
irradiation up to 450 nm, although the band gap of Nb2O5 is at 390 nm (3.2 eV). In the present study, we report that photocatalytic aerobic oxidations of various benzylic amines to imines take place over Nb2O5 with high yields at room temperature and atmospheric pressure. Nb2O5 exhibits a catalytic activity in high selectivity even under visible light irradiation as well as alcohol oxidation. Received: June 14, 2011 Revised: August 11, 2011 Published: August 15, 2011 1150
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ACS Catalysis
LETTER
Table 2. Aerobic Oxidation of Various Amines to Corresponding Imines Using Nb2O5a
a Reaction condition: Nb2O5 (100 mg), substrate (5 mmol), benzene as a solvent (10 mL), λ > 300 nm (entries 115) or λ > 390 nm (entries 10 150 ), oxygen pressure (1 atm). b Selectivities to corresponding imines. Figures in parentheses show selectivities to benzaldehyde. c Without catalyst. d Selectivities to N,N-dibutylformamide. e Selectivities to isoquinoline. f Selectivities to 3,4-dihydroquinoline. g Selectivities to 3,4-dihydroquinoline1(2H)-carbaldehyde. h Selectivities to indoline-1-carbaldehyde. i Selectivities to 10,11-dihydro-5H-dibenzo[b,f]azepine, respectively.
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Scheme 1. Schematic Illustration of Photo-Induced Electron Excitation: (1) Band Gap Excitation of Nb2O5 and (2) Excitation from a Donor Level Derived from Adsorbed Amine Species
Figure 1. (A) UVvis spectra of (1) Nb2O5, and (2) Nb2O5 with adsorbed benzylamine. (B) (3) Differential UVvis spectrum of (2) (1), and (4) action spectrum of benzylamine photooxidation over Nb2O5.
Photocatalytic activity and selectivity in the aerobic oxidation of benzylamine over various metal oxides are summarized in Table 1. TiO2, ZnO, and Nb2O5 show much higher yields of Nbenzylidene benzylamine than other oxides (Table 1, entry 13). Nb2O5 showed the highest yield per mole of catalyst (Table 1, entry 3). TiO2, which is an extensively used photocatalyst, also exhibited higher yield than Nb2O5 and ZnO. However the selectivity to N-benzylidene benzylamine was comparatively low because benzaldehyde was formed as a byproduct (Table 1, entry 1). The highest selectivity was obtained with Nb2O5 among the metal oxides tested. When benzylamine was photoirradiated in the absence of any catalyst, a very low yield was observed (Table 1, entry 11). The evolution of photogenerated products over Nb2O5 responded to illumination. Moreover, no product was detected in the dark with the Nb2O5 catalyst (Table 1, entry 4). These indicate that the photooxidation over the Nb2O5 catalyst was due entirely to a photocatalytic reaction. The oxidation of various amines including primary, secondary, and bicycloamine derivatives were examined by using the Nb2O5 photocatalyst (Table 2). Primary benzylamine derivatives bearing various functional groups (OMe, Me, H, Cl, and CF3) were converted to corresponding coupled imines with excellent yields (Table 2, entries 17). Electron-rich benzylamines (OMe and Me derivatives) were oxidized faster than electron-deficient ones (Cl and CF3 derivatives) (Table 2, entries 25). The reaction rate of the regioisomer increased in the order of orth < meta < para isomer, indicating the presence of a steric effect (Table 2, entries 2, 6, and 7). A primary aliphatic amine was also converted to the correspoding coupled imine, but the selectivity was lower than those of benzylic homologues (Table 2, entry 8). Secondary N-alkylbenzylamines (alkyl = Bn, Ph, iPr, and tBu) were also oxidized to dehydrogenated imines. (Table 2, entries 912). Relative high yields were observed within 20 h in the oxidations of N-isopropylbenzylamine and dibenzylamine. On the other hand, N-phenyl and N-tert-butyl derivatives were oxidized very slowly. The order of reaction rates corresponding to the bulkiness of the alkyl moiety (iPr > Bn . tBu > Ph) indicates that steric hindrance around the nitrogen atom is a key factor for the reaction rate. Benzaldehyde was formed as a byproduct in the oxidations of these secondary benzylic amines. The formation of benzaldehyde is attributed to the oxidative cleavage of the CN bond. In addition, a small amount of N-benzylidene benzylamine was also yielded in these cases. This represents an involvement of the CN bond cleavage, followed by coupling of the fragments. 1,2,3,4-Tetrahydroisoquinoline was smoothly converted to monodehydrogenated 3, 4-dihidroisoquinoline with high yield (Table 2, entry 13). In contrast, the rate of oxidation of 1,2,3,4-tetrahidroquinoline to
Scheme 2. Proposed Mechanisms of (A) Dehydrogenation of Amine, (1) Charge Separation, (2) Imine Formation; and (B) Dimerization of Amine, (3) Hydrolysis of a Primary Imine, (4) Condensation of Amine and Aldehyde
aromatized quinoline was much slower than the iso-isomer (Table 2, entry 14). In this case, a small amount of monodehydrogenated 3,4-dihidroquinoline as an intermediate product was also detected. The significant difference of the reaction rate between the tetrahydroquinoline isomers is generally observed in various catalytic systems.12,16,19,23 Indole was yielded with moderate selectivity in the oxidation of indoline. The Nb2O5 photocatalyst was reusable and showed the same conversion and selectivity without any pretreatment as the catalyst as prepared (see Supporting Information, Figure S1). The amine oxidations over Nb2O5 took place even under visible light (>390 nm) irradiation (Table 2, entries 10 150 ). Although the reaction rates were lower than that under UV (>300 nm) irradiation, comparable selectivities were obtained. In the absence of Nb2O5, oxidation of benzylamine did not proceed under visible irradiation (Table 2, entry 10 ). Nb2O5 shows an intense broad band at 300 nm because of band gap excitation, but no absorption in the region of λ > 390 nm (Figure 1A, trace 1). Indeed, the apparent quantum yield as a function of the wavelength of the incident light (action spectrum) does not agree with the UVvis spectrum of Nb2O5, and the reaction is responsive to light up to 460 nm (Figure 1B, plot 4). Therefore, this result indicates that the photoactivation mechanism of amine over Nb2O5 is different from the classical electron transfer mechanism found in semiconductor photocatalysis: the formation of an excited electron in the conduction band and of the positive hole in the valence band. We previously reported an analogous phenomenon; alcohol oxidation over Nb2O5 took place under visible light (>390 nm) irradiation. By means of UVvis, electron spin resonance (ESR), and FT/IR studies, the detailed reaction mechanism of alcohol photooxidation was revealed, and a unique 1152
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ACS Catalysis photoactivation mechanism by “in situ doping” was proposed, that is, the direct electron transfer from the O 2p orbital derived from adsorbed alcoholate species to the conduction band consisting of Nb 4d orbitals.22,24 In a similar fashion, amine oxidation over Nb2O5 with visible light may be attributed to a direct electron transfer from a donor level consisting of a N 2p orbital derived from adsorbed amine species (Scheme 1). Figure1A, trace 2 shows the UVvis spectrum obtained when benzylamine was added to Nb2O5. The absorption band at 300 nm extended to a higher wavelength region, and the differential spectrum before and after the adsorption of benzylamine showed a new absorption band at about 350 nm which was overlapped with the region of >390 nm (Figure 1B, trace 3). Since benzylamine itself exhibits no absorption in the region of >300 nm (See Supporting Information, Figure S2), the new absorption is ascribed to the surface complex consisting of adsorbed amine species and Nb2O5. Moreover, the differential spectrum shows a good agreement with the action spectrum. These results strongly suggest that the amine oxidation is triggered by light absorption by the surface complex consisting of adsorbed amine species and Nb2O5, in other words, direct electron excitation from the donor level derived from adsorbed amine species. On the basis of these results, we propose a possible reaction mechanism as shown in Scheme 2A. At first, dissociative adsorption of amine forms an amide (RR0 NNb) species. The electron excitation from the N 2p orbital of dissociatively adsorbed amine species (amide) to the conduction band results in the formation of an amide radical cation and reduced Nb(IV) (Step (1)). This radical cation intermediate is further oxidized to imine, and the electron is trapped to the other Nb(V) site (Step (2)). Then, the reduced Nb(IV) sites are reoxidized to Nb(V) by molecular oxygen. The proposed mechanism is essentially the same as that of alcohol photooxidation over Nb2O5.22 The photoactivation of amine is supposed to be more favorable than that of alcohol because of its donating ability. However, detailed the reaction mechanism is now under investigation and will be reported in the near future. In the oxidation of benzylamine, a corresponding primary imine was not detected. On the other hand, ammonia and benzaldehyde in addition to N-benzylidene benzylamine were observed, indicating that the hydrolysis of a primary imine took place. Moreover, when benzaldehyde was added to benzylamine in the absence of Nb2O5 in the dark, the condensation of benzaldehyde and benzylamine to N-benzylidene benzylamine immediately and quantitatively proceeded. These results indicate that a rapid dimerization takes place regardless of catalyst and photoirradiation; the produced primary imine is hydrolyzed to aldehyde and ammonia, followed by condensation of the aldehyde and the primary amine (Steps (3) and (4) in Scheme 2B).14,16 In conclusion, Nb2O5 can act as an efficient heterogeneous photocatalyst for the aerobic oxidation of various amines to the corresponding imines under atmospheric pressure at room temperature. This reaction takes place even under visible irradiation up to about 460 nm although the band gap of Nb2O5 is 390 nm.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details, recycle test, and UVvis spectrum of benzylamine. This material is available free of charge via the Internet at http://pubs.acs.org.
LETTER
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (T.S.), tanakat@ moleng.kyoto-u.ac.jp (T.T.). Funding Sources
This work was partially supported by Grants-in-Aid for Scientific Research on Priority Areas (No. 17034036, “Molecular Nano Dynamics” and No. 20037038, “Chemistry of Concerto Catalysis”) and Scientific Research (No. 19360365, “B”) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. K.T. is supported by the Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. T.S. acknowledges the Research Seeds Quest Program (Lower Carbon Society) (2010) under Japan Science and Technology Agency. S.F. thanks the JSPS Research Fellowships for Young Scientists.
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